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\begin{equation*}{eqnarray*}
\delta r_i &=& (\mbox{\boldmath $a$}_i/a) \cdot 
(\mbox{\boldmath $d$}_i-\mbox{\boldmath $d$}_{\rm C})\\
\delta\alpha_{ij} &=&- \, {\scriptstyle{1\over4}} \sqrt{2} \, 
\left[ \frac{(\mbox{\boldmath $a$}_i/a) \cdot 
(\mbox{\boldmath $d$}_i-\mbox{\boldmath $d$}_{\rm C})}{a} + 
\frac{(\mbox{\boldmath $a$}_j/a) \cdot 
(\mbox{\boldmath $d$}_j-\mbox{\boldmath $d$}_{\rm C})}{a} \right]^{\phantom I}\\
&~&- \, {\scriptstyle{3\over4}} \sqrt{2} \, \left[ \frac{(\mbox{\boldmath $a$}_i/a) \cdot 
(\mbox{\boldmath $d$}_j-\mbox{\boldmath $d$}_{\rm C})}{a} + 
\frac{(\mbox{\boldmath $a$}_j/a) \cdot 
(\mbox{\boldmath $d$}_i-\mbox{\boldmath $d$}_{\rm C})}{a}\right]^{\phantom I} ~ 
\end{equation*}
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